Method and device for producing expanded polyurethane moulded bodies

ABSTRACT

A method for manufacturing a foamed polyurethane molded article, that includes the steps of introducing an expandable polyurethane reactive mixture into a mold and evacuating the mold, expanding the reactive mixture expands so as to fill the mold, and exhausting gases liberated during the expanding step through one or more expansion openings disposed at one or more points of maximum height in a top mold region, each of the expansion openings being closeable by a needle valve disposed in a valve capillary. In addition the method includes sensing a temporal pressure characteristic in the valve capillary, controlling each of the one or more needle valves using the temporal pressure characteristic, so as to close the respective expansion opening in response to a pressure drop occurring when the expanding reactive mixture penetrates into the valve capillary, opening the mold, and ejecting the molded article.

FIELD OF THE INVENTION

The present invention is directed to a method for manufacturing foamedpolyurethane molded articles, whereby an expandable polyurethanereactive mixture is introduced into a mold; following the chargingoperation, the reactive mixture expands, filling the mold; gasesliberated during the expansion process being evacuated through expansionopenings located at points of maximum height in the top mold half;following the setting operation, the mold being vented and the moldedarticle being ejected.

BACKGROUND INFORMATION

It is known to manufacture foamed polyurethane molded articles byintroducing an expandable polyurethane reactive mixture into a moldseparated by a parting plane and by evacuating the mold via a vacuumchannel extending peripherally around the mold in the parting planethereof. To this end, a number of approaches have been discussed; see,for example, German Unexamined Patent Application DE-OS 15 04 278,German Patent Application DE 30 20 793 A1, European Patent ApplicationEPO 023 749 A1 and German Patent DE 197 01 728 C2. It is necessary, inparticular, to evacuate the mold in order to remove the gas present inthe mold to avoid the formation of voids. This requires designing themold halves in such a way that the parting plane lies at the highestpoint of the mold cavity, since, otherwise, a pocket will form out ofwhich the gas that is present can no longer can be exhausted. This couldbe counteracted by evacuating the mold cavity to a very low pressure ofless than 100 mbar, in particular of less than 50 mbar, before the foamrises, i.e., before the level of the foam in the mold cavity crosses theparting plane. However, such a low pressure in the mold cavity causesthe foam to initially expand quickly and vigorously, until stillsignificant amounts of blowing agent are released, with the result thatan only an irregular foam structure is formed. Another drawback ofevacuation via the mold parting plane is that the delivery rate via theparting plane is relatively low, particularly when the expandablereactive mixture is introduced into the open mold and evacuation is notcarried out until after the mold has been closed, so that

very long cycle times are required, and thus the time required forevacuation prolongs the cycle time.

The properties of polyurethane foam are substantially determined by thedensity of the finished foam and the material properties of the matrix.In particular, when water is used as a chemical blowing agent, carbondioxide being liberated by the reaction of the water with theisocyanate, it is necessary to fine-tune the formulation of theexpandable polyurethane reactive mixture in order to adjust the matrixproperties. Therefore, in principle, it is desirable to be able toproduce foams having different bulk densities using one singleformulation. To control the density of a foam while simultaneouslyretaining the formulation, in particular of the blowing agentconcentration, a control of the pressure in the foam mold is excellentlysuited; see, for example, European Patent Applications EP 0 023 749 A1and EP 0 044 226 A1, and German Patent DE 197 01 728 C2.

European Patent Application EP 0 023 749 A1 describes a method for thevacuum-assisted foaming of foam slab stock having slabstock sizes oftypically 2×1×1 m³ (in this regard, see page 7, line 12, of EP 0 023 749A1). To evacuate the foam slabstock mold, merely one line 19 or 20 isprovided, as is apparent from FIG. 1 of EP 0 023 749 A1. In order forthe mold to be evacuated within reasonable periods of time (i.e.,without substantially prolonging the cycle times which correspond moreor less to the curing times), lines 19 and 20 from FIG. 1 of EP 0 023749 A1 require a substantial cross section relative to the mold size. Inaccordance with the EP 0 023 749 A1, the inlet side of the lines is notdesigned to be closeable; rather, as can likewise be inferred from FIG.1 of the publication, stop valves 1 and 2 are only provided at the endof lines 19 and 20. If the device were operated in such a way thatnegative pressure prevailed at the end of the foaming process, the foamwould penetrate into lines 19 and, respectively, 20 and cure there.

In accordance with German Examined Patent Application DE 23 66 184 B1, afilter (denoted by reference numeral 46 in the only figure) is providedin the suction opening of the mold. The filter must be discardedfollowing each foaming process. In addition, the filter constitutes arelatively substantial resistance to flow, so that a rapid evacuation ofthe mold is not possible.

The German Patent Application DE 30 20 793 A1 also does not describe anyseparation between rapid evacuation and residual gas exhaustion. Rather,an evacuation gap is provided in the mold parting plane itself, that maysuffice for achieving a rapid enough evacuation for relatively flatmolded parts having a relatively flat volume in comparison to theperipheral size, and that is closed off by the penetrating foam mass.This results in a “flash formation” that typically must be subsequentlyremoved by hand.

A method of the type described at the outset is known from German PatentDE 197 01 728 C2. In accordance with DE 197 01 728 C2, the top mold halfhas a vent opening for evacuating the mold after the polyurethanereactive mixture is introduced. If the targeted operating pressure isreached in the mold, then the suction opening is closed. In addition tothe vent opening, the publication also discusses evacuating the mold viathe mold parting plane; this is associated with the known disadvantageof a flash formation. To avoid the formation of voids, in particularwhen using molds having contoured mold cavities, the German Patent DE197 01 728 C2 also discusses configuring so-called expansion channels inthe top mold half at points of maximum height that are likewiseconnected to the vacuum system and through which the gases liberatedduring the expansion process are to be evacuated. It is provided in thiscontext that the polyurethane foam penetrate into the channels uponreaching the top mold half, that it cure there, and thereby seal thechannels. The expansion openings are provided with cleaning pushrods,which are actuated after the molded article is ejected, in order to beable to remove polyurethane still present in the channels. The drawbackof the known method is that the molded article may have sprue-likeprojections produced by the penetration of the polyurethane into thechannels that must be removed, together with the flash formations, in acostly secondary machining operation. Also to be taken intoconsideration when working with spatially greatly extended molded foamarticles having long flow paths is that the foam front reaches theexpansion channels at very different points in time. Consequently, atthe channels that it reaches first, the polyurethane reactive mixture isstill very flowable, since the polymerization reaction is still not yetfar advanced, and can thus penetrate far into the channel. Thus, thereis also the danger of the expansion opening being blocked.

SUMMARY OF THE INVENTION

The object of the present invention is to further develop a method ofthe type described at the outset in such a way that, even when workingwith long flow paths and unfavorable conditions with respect to the moldparting plane, it will be possible to produce void-free articles withinshort cycle times. This objective is achieved by a method having all ofthe features set forth in claim 1. A device according to the presentinvention is described in claim 9. The dependent claims relate toadvantageous embodiments of the present invention.

In accordance with the present invention, in a method for manufacturingfoamed polyurethane molded articles, in which an expandable polyurethanereactive mixture is introduced into a mold; following the chargingoperation, the reactive mixture expands, filling the mold; gasesliberated during the expansion process being evacuated through expansionopenings located at points of maximum height in the top mold half;following the setting operation, the mold being vented and the moldedarticle being ejected; the expansion openings are formed by needlevalves which are controlled in such a way that they close immediately inresponse to the first ingress of the foam front into the valvecapillary.

A device according to the present invention having a mold having a topmold region, having a suction opening for evacuation purposes and adevice for venting the mold, as well as having expansion openings at oneor more points of maximum height in the top mold region has thedistinguishing feature that the expansion openings are formed by needlevalves that are controllable in such a way that they close immediatelyin response to the first ingress of the foam front into the valvecapillary.

It turns out that when a needle valve is used instead of the expansionopening known from the German Patent DE 197 01 728 C2, it is possible tocompletely prevent the formation of sprue-like projections. This merelyrequires controlling the valve in such a way that it closes immediatelyin response to the first ingress of the foam front into the valvecapillary. At that moment, the polyurethane foam is clearly not yetreacted to completion and is able to be easily pushed out of thecapillary by the valve needle that is descending in the capillary toclose the valve, before a sprue-like projection can form. Thiseliminates the need for the costly secondary machining process knownfrom the related art.

This result is all the more surprising, as a very complex geometry forthe expansion openings and the cleaning pushrods is provided in theGerman Patent DE 197 01 728 C2, which is intended to ensure, on the onehand, that the polyurethane is not able to penetrate too far into thevacuum system and, on the other hand, that the cleaning process is to besimplified for the expansion opening. This difficulty is able to becompletely circumvented by the use in accordance with the presentinvention of a needle valve. By closing the valve in response to thefirst penetration of the foam front into the capillary, any furtheradvance of the polyurethane into the vacuum system is completelyprevented. Due to the fact that polyurethane that is already present ispushed out of the valve capillary already in response to the closing ofthe valve, the need is not only eliminated, as already mentioned above,for the secondary machining of the molded foam article, but also for thecleaning process required in the known method in which polyurethaneresidues must be partially removed, even by using boring tools. Thus,another important simplification of the method is derived herefrom.

A further benefit is derived from the possibility of a process controlvia the valves. While the known method does not provide for or permitintervening in the expansion process, in the method according to thepresent invention, greatly differing variants of a process control areconceivable due to the possibility of driving the valves in any desiredmanner. Thus, for example, the pressure may be adjusted as a function ofthe formulation of the reactive mixture. Also conceivable are a controland/or regulation of the evacuation pressure during the expansionprocess or the setting of different evacuation capacities when aplurality of expansion valves is used for spatially extended molds.

Needle valves, as are used here, are prevalent in many fields ofapplication of the related art. Surprisingly, it turns out thatconventional needle valves may be used, without requiring any specialadaptation to the use according to the present invention. To cleanlypush out a polyurethane reaction mixture that is not yet reacted tocompletion using the valve tappet of the needle valve, it is merelynecessary that the needle valve be manufactured with sufficientprecision.

The manufacture of polyurethane foam is generally known and is notdescribed in greater detail here. The method and, respectively, thedevice according to the present invention are not limited to specialprocess variants, in particular formulations or process controls, buthave universal applicability. Thus, the reactive mixture may be pouredboth into the open as well as into the closed mold, without limitinguniversality, it being possible for the foaming process to be initiatedboth by charging with carbon dioxide or another propellant, such as air,nitrogen, etc., as well as with conventional blowing agents, such aswater, or a combination of blowing agents. Depending on the processvariant, the mold may then be evacuated via the vacuum relief valves toa minimum negative pressure of 300 mbar. Following the foaming andsetting of the reactive mixture, the mold is finally vented and opened.The molded article is ejected, if required, with the assistance ofcompressed air.

One preferred embodiment of the present invention provides for theliberated gases to be exhausted during the expansion process, but alsofor the mold to likewise be evacuated via the needle valves. Thus, theneed is eliminated for an additional suction opening, as it is forevacuating via the mold parting plane. Since, in the method according tothe present invention, the expansion opening is not sealed by curing afoam plug in the vacuum channel as in the known method, whichnecessitates a maximum possible diameter for the capillary, sinceotherwise the polyurethane would penetrate too far into the vacuumsystem before it cures, but rather simply by closing the valve, thegeometric dimensions of the capillary may easily be adapted to thisadditional function.

Capillary diameters of between 0.2 mm and 2 mm are preferred. If thecapillary diameter is selected to be smaller than 0.2 mm, the volumetricflow of reaction gases that is able to be evacuated through thecapillary is greatly diminished, since the pressure loss is inverselyproportional to the capillary diameter. As a result, either the cycletime is increased, or a larger number of valves is required.

On the other hand, if the capillary diameter is selected to be largerthan 2 mm, then, due to the minimal pressure loss, it becomes moredifficult to detect the change in the flow through the capillary as thefoam penetrates that is used to determine the valve closing.Compensating by lengthening the capillary (in accordance withHagen-Poiseuille, the pressure loss is proportional l/d (=length todiameter of the capillary) considerably influences the size, which is tobe kept small in accordance with the installation possibilities onand/or in the mold.

In another preferred embodiment of the present invention, to control theneedle valve(s), a chemical and/or physical quantity that changesrapidly in response to the ingress of the foam front into the valvecapillary is recorded, and the needle valve is controlled as a functionof the time characteristic of this quantity. The advantage of this typeof control is that the valve closes immediately and autonomously inresponse to penetration of the foam front into the capillary.

As a control variable, preferably the temporal pressure characteristicin the valve capillary is sensed, the control being designed in such away that the valve closes immediately in response to the pressure dropin the capillary occurring when the foam front penetrates into the valvecapillary. In accordance with the Hagen-Poiseuille principle, thepressure loss in the capillary is dependent on the viscosity of themedium flowing through. In response to penetration of the foam front,the pressure loss increases by approximately a factor of 10⁵-10⁶ becauseof the difference between the viscosity of polyurethane and that of air.For that reason, in accordance with the present invention, a pressuresensor is provided for recording pressure whose output signal is fed toa control unit which, in turn, converts it into a control signal formoving the valve needle. As pressure sensors, the generally knownpiezoelectric pressure sensors are suited, for example.

Another variable for controlling the valve in the sense described abovemay also be the flow rate through the valve capillary, for example,which, for the same reasons as described above, decreases by acomparable factor in response to penetration of the foam front into thecapillary. However, the method according to the present invention is notlimited to these especially suited control variables which are mentionedexemplarily.

Other preferred embodiments of the present invention provide for theneedle valves to be used to assist in the removal process, in additionto venting the mold and/or acting upon the mold with compressed air.This also reduces the number of component parts, so that cost advantagesare attained.

A system for manufacturing polyurethane molded articles is furthersimplified by supplying a plurality of needle valves with negativepressure and/or with compressed air from a shared media supply. Anegative pressure is nevertheless able to be individually adjusted ateach needle valve due to the autonomous control provided for each valvevia a proportional valve.

The present invention is explained in greater detail in the followingwith reference to the figures, which show:

FIG. 1: in a schematic longitudinal sectional representation, a needlevalve that is controllable in accordance with the present invention viaa pressure sensor;

FIG. 2: in a schematic longitudinal sectional representation, a devicein accordance with the present invention having a needle valve;

FIG. 3: in a schematic longitudinal sectional representation, a devicein accordance with the present invention having a plurality of needlevalves and a shared negative pressure and/or positive pressure supply;

FIG. 4: schematically, the characteristic curve of the pressure in thevalve capillary during the foaming process and upon penetration of thefoam front into the valve.

FIG. 1 illustrates a needle valve 1 suited for implementing the methodaccording to the present invention. Needle valve 1 essentially includesa housing 2, a valve needle 3, a valve seat 4, as well as a capillary 5.Also shown is a pressure sensor 6. As soon as the foam front penetratesinto valve capillary 5, the pressure in capillary 5 drops sharply. Thispressure drop is detected by pressure sensor 6 and converted via acontrol unit (not shown in the figure) into a control signal for valveactuation 7. In response to this control signal, valve needle 3 descendsand valve 1 closes. In the process, the polyurethane that has alreadypenetrated into capillary 5 is pushed out through valve needle 3.

FIG. 2 shows one preferred embodiment of a device in accordance with thepresent invention. A mold 10 is shown having a top 10 a and a bottommold half 10 b. The two mold halves are separated from one another bymold parting plane 10 c. Mold parting plane 10 c preferably has a vacuumseal. Also illustrated is a needle valve 1 situated in top mold half 10a. Needle valve 1 communicates via a four-way valve 11 both with anegative pressure and a positive pressure source (not shown here), aswell as with the atmosphere. This system not only permits the gasesliberated during the expansion process to be exhausted via needle valve1, but also mold 10 to be evacuated or vented, as well as acted upon bypositive pressure. The output signal from pressure sensor 6 is used forcontrolling the valve actuation of needle valve 1. In response to apressure drop caused by polyurethane penetrating into valve 1, valveneedle 3 is lowered and valve 1 is closed, the already penetratedpolyurethane being pushed out.

FIG. 3 depicts the connection of a plurality of valves 1 to a sharedmedia supply. The arrangement of a plurality of valves may be usefulwhen working with very extended, flat-shaped molds, in order tocompensate for the pressure loss caused by long flow paths, byindividually adjusting the negative pressure at valves 1. Anotherpossible application is for molds having a complex geometric shape, inparticular having a plurality of points of maximum height, in order toprevent the formation of voids. Other possible applications areconceivable. Discernible in the figure are a plurality of needle valves1 having pressure sensors 6, which are situated in top mold half 10 a ofmold 10. Needle valves 1 are each connected via a four-way valve 11 toshared supply lines leading to a negative and/or positive pressuresource 14. In this case, the negative pressure source is constituted ofa vacuum vessel 12 which is evacuable by a vacuum pump 13. Moreover,each four-way valve 11 also has an outlet to atmosphere. Four-way valve11 is preferably a proportional valve, to render possible an individualadjustment of the negative pressure at each valve 1, in spite of ashared media supply.

FIG. 4 shows the pressure characteristic including thecontrol-triggering pressure drop occurring when the foam front reachesthe valve capillary. As is discernible, the pressure drops suddenly inresponse to penetration of the foam front (instant A). The slight dropin pressure that is already observable beforehand, is attributed to theincrease in viscosity of the reaction mixture during the advancingpolymerization reaction. The steep pressure drop in response topenetration of the foam front is sensed by the pressure sensor and usedas a triggering factor for closing the valve.

1-16. (canceled)
 17. A method for manufacturing a foamed polyurethanemolded article, the method comprising: introducing an expandablepolyurethane reactive mixture into a mold and evacuating the mold, themold having a top mold region; expanding the reactive mixture expands soas to fill the mold; exhausting gases liberated during the expandingstep through one or more expansion openings disposed at one or morepoints of maximum height in the top mold half, each of the expansionopenings being closeable by a needle valve disposed in a valvecapillary; sensing a temporal pressure characteristic in the valvecapillary; controlling each of the one or more needle valves using thetemporal pressure characteristic, so as to close the respectiveexpansion opening in response to a pressure drop occurring when theexpanding reactive mixture penetrates into the valve capillary; openingthe mold; and ejecting the molded article.
 18. The method as recited inclaim 17, wherein the evacuating of the mold is performed using the oneor more the needle valves.
 19. The method as recited in claim 17,wherein the introducing step is also performed using the needle valves.20. The method as recited in claim 17, wherein the ejecting of themolded article is performed using the needle valves to act upon the moldwith compressed air.
 21. The method as recited in claim 17, wherein eachof the one or more needle valves is supplied with at least one ofnegative pressure and compressed air from a shared media supply.
 22. Themethod as recited in claim 17, wherein the one or more needle valvesincludes a plurality of needle valves and further comprising adjusting anegative pressure to each of the needle valves individually.
 23. Adevice comprising: a mold having a top mold region and configured toreceive an expandable polyurethane reactive mixture; a suction openingconfigured to evacuate the mold; a vent configured to vent the mold; oneor more expansion openings disposed at one or more points of maximumheight in the top mold region, each of the expansion openings beingformed by a needle valve disposed in valve capillaries, wherein theneedle valves are closable in response to a pressure drop occurring whenthe expandable polyurethane reactive mixture penetrates into the valvecapillary.
 24. The device as recited in claim 23, wherein the suctionopening is likewise formed by a respective one of the needle valves. 25.The device as recited in claim 23, wherein the needle valve enable themold to be acted upon with compressed air.
 26. The device as recited inclaim 23, further comprising a four-way valve, and wherein the at leastone needle valve is connected in series to the four-way valve, thefour-way valve capable of establishing a communication to at least oneof a negative pressure source, a positive pressure source andatmospheric pressure.
 27. The device as recited in claim 26, wherein thefour-way valve is a proportional valve.
 28. The device as recited inclaim 23, further comprising a media supply, and wherein the at leastone needle valve includes a plurality of needle valves, each suppliedwith at least one of negative pressure and compressed air from the mediasupply.